Crystal chemistry and thermal expansion of Cd0.5Zr2(PO4)3 and Cd0.25Sr0.25Zr2(PO4)3 ceramics

MateaidsResearchBulletin.Vol. 32, No. 1, pp. 15-23.1997 Copyright0 1996 ElsevierScienceLtd printedin theUSA. All rightsreserved 0025-5408/97 $17.00 +.OO

PH SOO255408(96)00162-6

CRYSTAL

CHEMISTRY

AND THERMAL EXPANSION Cdo.&ro..uZr2(PO& CERAMICS

OF Cdo.sZrz(PO& AND

R Brochu’, M. El-Yacoubi’, M. Lou&‘, A. SerghinP, M. Alami’ and D. Lou&r’ ’ Laboratoire de Chimie du Solide et Inorganique Mol6culah-e (URA CNRS 1495), Groupe de Cristallochimie, Universite de Rennes, Avenue du G&&al Leclerc, 35042 Rennes cedex, France ’ Laboratoire de Chimie du Solide, Departement de Chimie, Faculte des Sciences, Rabat, Maroc (Refereed) (Received October 7, 1996; Accepted October 10, 1996)

ABSTRACT Cdo.sZrz(P04)s (CdZP) and the partially substituted phase Cd,&3ro.zsZrz(P04)3, prepared by the sol-gel route, belong to the NZP family. The structure of CdZP (S. G. Z@ has been established by the Rietveld method. The thermal behavior of CdZP, investigated by X-ray powder diffraction, is anisotropic, i.e., the structure expands along c axis and contracts along a axis. Dilatometric measurements and SEM micrographs of ceramics, sintered with 5% ZnO, show that CdSrZP presents the advantage to decrease microcracking, and then anisotropy, occurring during the heating cycle. The thermal expansion coefftcient abt,/k of the ceramic CdSrZP is very small (0.6 x 10” “C’). KEYWORDS: A. inorganic compounds, D. microstructure, D. thermal expansion

A. ceramics,

C. X-ray diffraction,

INTRODUCTION The ceramic materials referred to as NZP m&&(PO&] and CZP [Ca&3$PO.&], with low coefficients of thermal expansion (CTE), are developed for potential applications in high temperature technologies (l-5). A common feature for most members of the family is the highly anisotropy of the axial thermal expansion, which limits their use due to microcracking occurring during the heating cycle (6,7). However, the magnitude of anisotropy can be tailored, for example, by using solid solutions from materials with 15

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R. BROCHU et al.

Vol. 32, No. 1

opposite thermal expansions, as Ca1_&xZr4(P04)6 or Sr1_&Zr4(PO& (8,9). On the other hand, some structural studies have been reported in order to identify the factors which can influence the thermal expansion of these compounds (10,ll). The present study deals with cadmium-based ceramics, C&.sZi$PO& and Cdo.2sSro.zsZrz(P0&, with NZP type. Synthesis, structural, sintering, and thermal expansion properties are reported and are compared with those of materials belonging to the MuO.sZrz(PO.&series. EXPERIMENTAL Powder

Preparation. The samples of C&.sZrz(PO& (CdZP) and Cdo.25Sro.&r~(P0.+)3 (CdSrZP) were prepared by the sol-gel route, instead of the usual solid state reaction (12). The starting materials CdC03 and/or SrCO1, ZrOC12-8H20, and (NH&HP04 were dissolved in slightly acidified or aqueous solutions. The solution of ammonium phosphate was added dropwise to a mixture of zirconium oxychloride and carbonate solutions, under constant stirring, from which a gelatinous precipitate was formed. The gel and floating solution mixture were maintained at 80°C for 24 h, then the residue was heated progressively to 900°C. The formation of a single phase, type NZP, was confirmed by powder difliaction, though traces of Zr20(PO& were sometimes detected. Sintering and Characterization. The powders were pelletized (6 mm diameter and 3 mm thick) under a pressure of 125 MPa and sintered in air at 1000 and 1lOO’C, with the aid of ZnO as an additive in a proportion varying from 1% to 5%, for 1 to 10 h. The green density of the pellets was determined from geometric measurements. After sintering, the specimens with density greater than 94% (or even 98% for CdZP) of the theoretical density were used as cylindrical bars for the dilatometric measurements in the range 20-800°C (heating rate S”C/min) with a dilatometer NETZSCH 402EP. The microstructural characterization was carried out by examining the polished surface using scanning electron microscopy (SEM JEOL JSM 6400). X-ray Diffraction.

The structure of CdZP was studied from monochromatic X-ray powder diffraction data collected with a Siemens diffractometer (CuK~li) (13). The pattern was scanned in steps of 0.02°(2q) (counting time: 60 s step-‘), over the range lO-lOO’(20). The presence of line broadening was detected from pattern fitting technique. The Rietveld refinement was carried out with the program FULLPROF (14). A diffractometer equipped with a graphite diffracted-beam monochromator (CUKCQ) was used for the hightemperature diffraction study of CdZP (15). Data were collected at 50°C intervals from room temperature to 6OO’C.The refinement of the unit cell parameters was carried out with the program NHS*AIDS83 (16). RESULTS AND DISCUSSION

The powder diffraction patterns of Cdo.5Zrz(P04)s and Structure Analysis. Cdo.25Sro.zsZrz(P04),are characteristic of the NZP structure. They can be indexed assuming a rhombohedral cell, the parameters of which expressed in the hexagonal system are g@en in Table 1. The variation of parameters ah and ch from CdZP to CdSrZP and SrZP (expansion along c and contraction along a) is in accordance with the substitution effect generated by

CADMIUM ZIRCONIUM

Vol. 32, No. 1

TABLE

Crystallographic

Data for CQ.&r,Zr@‘G&

x

rXM”3(4

ah (4

0.25 0.5

0.95 1.065+ 1.18

8.8386(8) 8.760(2) 8.704(2)

-0

17

PHOSPHATE

1

(x = 0,0.25,0.5) ch(4

22.286(3) 22.74( 1) ! 23.38(l)

Phosphates (RT)

v(A31

&z/c(dm31

1508 1511 1533

3.46 3.41 3.32

*Average ionic radius: (red + r&2

increasing, cation size in the Ml site (17). The space grout is different of that of NaZrz(P04)3 (R%). In.deed, a few weak reflections with indices hhOl(l = 2n + l), not allowed in the R& space group, agree with the R?or R32 space groups. The crystal structure of CdZP has been investigated by the Rietveld method, assuming the two space groups, with the atomic coordinates of NaZrz(P04)3 (NZP) (18) as starting structural parameters. The objective was to localize precisely the Cd*+ cations, similarly to strategies already reported in this family (19,20). Three situations were considered:

(0 (ii) (iii)

in the space group R? the cations Cd*+ are located in the sites 3a or 3b with a complete occupation rate. Qalso, Cd*’ can occupy both 3a and 36 sites with a half occupancy factor. in the R32 space group, the 6c site (0, 0, ~0.25) tilled by Cd’+ with a half occupancy factor.

clo*POo I Il,,,,l II I ,,I

I I IU II II I 11111II Ia

TWO-THETA 100.000> llllli IIS II I lllll lllll IM A III 11liIII llll mllslllullalmal IIM IIllIIIIImPIImIIR~lll II IllI 11111111 llllllllllN lllillllPIIllllm 11111 11111111111 I IIIIIIIIIUIlllAllIllllllallrlll~

++ FIG. 1 The fmal Rietveld plot of Cdo.sZr@O,&. The vertical markers show positions Bragg reflections, lower part for CdZP, ,upper part for the impurity Zr20(P0.&.

calculated

for

R. BROCHU et al.

18

Atomic Coordinates Atom Cd Zrl Zr2 P 01 012 02 022

3b 6c 6c 18Y 18s 18f 18f l8f

TABLE 2 and Isotropic Temperature X

Y

0 0

0 0

0.2&l) 0.179(3) 0.001(2) 0.198(2) -0.166(2)

*0 atoms have been constrained

Vol. 32, No. 1

Factors (A’) for CdZP* I

-0.0002(1) -0.017(3) -0.187(2) 0.167(2) -0.194(2)

B

0.5 0.1370(2) 0.6482(2) 0.2490(5) 0.1906(9) 0.6952(9) 0.0834(9) 0.5845(7)

2.8(l) OS;(k) 0.37(6) 1.1(l) 1.3(l) 1.3(l) 1.3(l) 1.3(l)

to have the same isotropic B factor.

The reliability factors, R&RF, obtained with these three models were 4.9313.41, 9.W6.58 and 12.80/X.41, respectively. They indicate that case (i) is the most probable, as also shown by the consistency of the interatomic distances, on the contrary of the unlikely values obtained with the other hypotheses. Figure 1 shows the final Rietveld plot for the refinement of the CdZP structure in the space group R?. Some discrepancies on the difference curve can be explained from the anisotropic line broadening.The atomic coordinates are given in Table 2. Selected distances and angles are listed in Table 3. The structure of CdZP is similar to that of SrO5Zr2(PO& and Ca&Zr~(PO& (lo), which is consistent with the possibility of solid solutions between the two end compounds Cdo.sZrz(P04)s and Sro.sZrz(PO.&. This structure is based on a 3D framework built from octahedra [ZrOh] and tetrahedra [POJ sharing comers. The Cdz’ cations occupy, with a 50% occupancy rate, Ml site with an order Cd*+- along c axis. The distances Cd-O, 2.47 A, are about 5% longer than the sum of ionic radii according to Shannon (21). This is an indication on the stability of the covalent framework Zrz(PO.&- which accommodates Cd*’ as guest. The [ZrOa] and [PO41 polyhedra are slightly distorted, e.g., the P-O distances are in the range 1.52-1.57 A (mean: 1.55 A). In the CdSrZP phase, there is a random substitution of Cd*’ and Sr*’ on site Ml. The variation with temperature T(“C) of the cell Anisotropic Thermal Expansion. parameters ah and ch for the CdZP ceramic, shown in Fiv 2, can be fitted by a secondTABLE 3 Selected Distances (A) and Angles (“) for CdZP 022-Cd-022

68.3(2) (x6)

Cd

022

2.47(2) (x6)

Zrl

01 02

2.05(2) (x3) 2.02(2) (X3)

Zr2

012 022

1.96(2) (x3) 2.14(2) (x3)

01-P-012 01-P-02

11 l(3) 113(3)

P

01 012 02 022

1.57(3) 1.52(2) 1.56(2) 1.56(2‘)

0 l-P-022 012-P-02 0 12-P-022 02-P-022

107(3) 108(2) 109(2) 1 lO(2)

112(2) (x6) 180.0(8) (x3)

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CADMIUM ZIRCONIUM PHOSPHATE

19

22.80

0

21.80 0

200

FIG. 2 Variation of a,, and ch parameters With tempeI?Iture,

for

BE IO 600 .,. (oC) 1

400

Cdo.5Zrz(PC&.

order polynomial, whose coefficients PO, PI andp2 are given in Table 4. It corresponds to the anisotropic behavior usually observed in the NZP family, as a consequence of a cooperative rotation of the [ZrOa] and [PO41 polyhedra in the skeleton of the structure. (22). The parameter ah decreases from 8.8386(8) to 8.8150(8) A, while CI, increases regularly from 22.286(3) A to 22.451(3) A, as the temperature increases from room temperature to 7OO’C. The variation of the cell volume is very small in this temperature range. The thermal expansion coefficients CX~ [= (l/p)(dp/dT)] at 5OO’C are CCI=-3.2 x IO” ‘C-’ and ac = +l 1.4 x 10” “C-l, i.e., an average expansion coefficient G [= (201, + c&)/3] = 1.6 x 10” “C-l. The axial anisotropy at 500°C is large: Act (= 1Q - d ) = 14.6 x 10” YY’. These values of CTE are compared in Table 5 to those reported for some compounds in the series Mo.5Zrz(P04)s (M = Ca, Sr) (23). CdZP behaves as Cao.5Zrz(P04)~ for the variation with temperature of OcI and a, coefficients, with a significant anisotropy Aa. On the other hand, the positive average volume expansion coefficient a, is close to the corresponding value for SrZP, for which the anisotropy is however smaller. This result explains the reason why the solid solution Cdo.&XZrz(PO4)~ (x = 0.25) has also been studied to try to minimize the anisotropy observed in the CdZP ceramic. Sintering and Microstructure of CdZP and CdSrZP Ceramics. Both ceramics require the aid of ZnO as additive to the pure phases for a correct sintering. ZnO acts through the formation of an intergranular zinc-phosphatc+based liquid (24). An optimum concentration of 5% promotes a higher densitication (98-99%) when CdZP ceramic is heated at 1000°C for 1 h, and 94% for CdSrZP with the same sintering conditions. The SEM micrographs (Fig. 3) of the sintered CdZP ceramics show a rather good densification with gram sizes in the range 124 mm, with also a few larger grains. The intergranular porosity can be observed

Coefficients

TABLE 4 of the Second-Order Polynomialp (A or A3) (=po +p,T + pzp) Unit-Cell Parameters Against Temperature T (“C) Coefficient PO PI P2

ah

ch

8.8401

22.2803

-0.557

x 10-4

0.274 x lo-’

0.0002 0.56 x 10-7

Used to Fit the

V

1507.9 -0.0055 0.1335 x lOA

R. BROCHU ef al.

Vol. 32, No, 1

(b)

FIG. 3 SEM micrographs of CdZP and CdSrZP ceramics, sintered with 5% ZnO at 1000°C for 1 h, except for c (10 h). (a) CdZP. (b) intergranular film in CdZP. (c) microcracks in CdZP. (d, e) CdSrZP. cf) backscattered electron image for CdSrZP. as well as trapped pores and a thin film between the grains. Microcracks are mainly for the ceramics treated at 1lOOT or sintered for a greater time (10 h), grain size, inversely proportional to (Aa)‘, exceeds a critical value according to and Bradt (25). For CdSrZP ceramic the grain size (2-3 pm) is more homogeneous and no negligible microcracks are detected. Backscattered electron image in SEM (Fig.

detected, for which Cleveland visible or 3) reveals

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O.Wf

CADMIUM ZIRCONIUM PHOSPHATE

21

*+* *=**+*+**##+* ******* 1

$ 0.000

0.005 L. -0.003

,

0

0.005

0.003

0.000

t

200

,

8

400

,

n

6W

,T('C)

0

600

1

0.003

1

41++++++ +

+

0.000

+++++=z*+

-0.003

-0.005 +

0

,

,

200

,

, 4w

! *++ +

++++++*++

, 600

,

, 600

-WI

’

600

600

d

++++

+++++++++

+

+

+

-0.003

,

400

N-/L

0.005

AL/L

200

++

+++*+

+++

T(“C)

-0.005-A 0

200

FIG. 4 Thermal expansion behavior of CdZP (a) and CdSrZP (b) ceramics, at 1000°C for 1 h and for CdZP sintered for 5 h (c) and 10 h (4.

4w

600

600

sintered with 5% ZnO

the presence of two minor phases: ZrOz in the form of small clear rounded second phase as light stick-like grains assumed to be zirconium phosphate.

grams and a

Dilatometric Measurements of CdZP and CdSrZP. Dilatometric analysis was carried out on three samples of CdZP sintered at 1OOO’C with 5% of ZnO for 1 h, 5 h, and 10 h. Until 6OO”C, AL/L is close to zero for the fast specimen and the macroscopic thermal expansion of CdZP is very weak and slightly negative (CC~B= -0.6 x IO-” “C-’ in the range RT-6OOOC). It decreases regularly with the sintering time (Fig. 4) and the thermal expansion curves show hysteresis loops due to microcracking as expected. Additional sintering tests with ZnO additive up to 20% revealed a higher densification, but abdk was increased (~2 x 10d ‘C-‘). This feature could be related to the presence of positive-CTE phases in the mixture obtained by reaction between ZnO and CdZP. The dilatometric curve of CdSrZP, sintered for 1 h with 5% ZnO, shows an expansion coefficienl: c&Urk weak and slightly positive, 0.6 x le OC-’ at 500°C. The curve AWL versus T shows a less pronounced hysteresis phenomena (Fig. 4), suggesting that microcracking is negligible in this material, with respect to CdZP, and consequently a significant reduction of

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R. BROCHU et al.

TABLE

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5

CTB at 5OO“Cfor Some MO5Zrz(PO& Compounds (M = Cd, Ca, Sr, Ca-Sr, Cd-Sr)

(sintered S. G. %(X 1O”OC’) a, (x 10” OC’) am(x 10d OC’) Aa (x 1o-6 0C-‘) a,,,,/&(x 1O-6 OC-‘)

CdZP 1 h, 5% ZnO) R3

CaZP (8) R3

-3.2 (-4.9) 11.4 (10.6) 1.6 (0.3) 14.6 -0.6

-5.1 9.9 4.1 15 -1.6

SrZP (8)

CaSrZP (8)

CdSrZP

R3

R5

R5

3.6 (3.8)O -1.2 (-2.8)” 2 4.8 (6.6)” 3.lb

-0.7 1.1 -0.1 1.8 1.4

0.6

Note. Values in parentheses were obtained assuming a linear dependence. u from ref. 11; b from ref. 9.

anisotropy. The behavior of CdSrZP (Table 5) is interesting since both expansion coefftcient and anisotropy are moderate. CONCLUSION The phosphates Cdo.5Zr2(P0& and C&.2s Sr0.25(PO& have been prepared by the sol-gel route. They crystallize with the NZP structure-type with the space group RT as demonstrated by the structural investigation of CdZP. The site Ml is half filled by the Cd’+ cations. The microstructural study has shown that well densified ceramics can be obtained by sintering at lOOO”C,by using 5% of ZnO. The thermal expansion of CdZP is small at 500°C: a, = 1.6 x 10m6OC-’ from X-rays and olb,,fk=-0.6 x 10e6 OC-’ from dilatometty. However the anisotropy remains significative (~14 x lo6 “C-‘) and explains the formation of microcracking. The partial substitution Cd2”/Sr2’ in CdSrZP allows to minimize microcracking and to decrease the anisotropy, as clearly shown from the dilatometric curve with a limited hysteresis loop and from the observation of the SEM micrographs. The results described in this study, mainly for CdSrZP ceramic, can be compared to those reported for Cao.zsSro.25Zr2(P0.&(CaSrZP) and make this material an additional candidate for potential applications. ACKNOWLEDGMENT The authors thank Mr. J. Le Lannic (CEMB, Rennes) for help in the SEM study. REFERENCES 1. R. Roy, D.K. Agrawal, J. Alamo and R.A. Roy, Mar. Rex Bull. 19,471(1984). 2. T. Oota and 1. Yamai, J. Amer. Cerum. Sot. 69, 1 (1986). 3. “Low Expansion Materials,” in Ceramic Trans., Vol. 52, ed. D.P. Stinton and S.Y. Limaye, Amer?Ceram. Sot., Westerville, OH (1995). 4. C.Y. Huang, D.K. Agrawal, H.A. McKinstry and S.Y. Limaye, J. Muter. Res. 9,2005 (1994). 5. S.Y. Limaye and R. Nagesnaran, Development of NZP Ceramics Based Cast-in-Place Diesel Engine Port Liners, Final Report LoTEC Inc., West Wally City, UT, Nov. 1994.

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6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

17. 18. 19. 20. 2 1. 22. 23. 24. 25.

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23

1. Yamai and T. Oota, J. Amer. Ceram. Sot. 70, 585 (1987). D.M. Liu and J.J. Brown, Muter. Chem. Phys. 33,43 (1993). S.Y. Limaye, D.K. Agrawal, R. Roy and Y. Mehrotra, J. Mater. Sci. 26,93 (1991). C.-J. Chen, L.-J. Lin and D.-M. Liu, J. Mater. Sci. 29, 3733 (1994). J. Alamo and J.L. Rodrigo, Solid State Zonics 63-65,678 (1993). D.-M. Liu, L.-J. Lin and C.-J. Chen, J. Appl. Cyst. 28, 508 (1995). A. El-Jazouli, M. Alami, R. Brochu, J.M. Dance, G. Le Flem and P. Hagenmuller, J. Solid State Chem. 71,444 (1987). D. Lou& and J.I. Langford, J. Appl. Cryst. 21,430 (1988). J. Rodriguez Carvajal, in Collected Abstract of Powder Diffraction Meeting, p. 127, Toulouse, France (1990). R. Brochu, M. Lou&, M. Alarm, M. Alqaraoui and D. Lou&, Mat. Res. Bull., in press. A.D. Mighell, C.R. Hubbard and J.K. Stalick, NBS*AZD.WO: A FORTRAN Program for Crystallographic Data Evaluation, Nat. Bur. Stand. (US) Tech. Note 1141 (1981). [NBS*AIDS83 is an expanded version of NBS*AIDS80]. J. Alamo, Solid State Zonics 63-65,547 (1993). L.O. Hagman and P. Kierkegaard, Acta Chem. Stand. 22, 1822 (1968). S. Senbhagaraman, T.N. Guru Row and A.M. Umarji, Solid State Comm. 71,609 (1989). 0. Mentre, F. Abraham, D. Deffontaines and P. Vast, Solid State Zonics 72,293 (1994). R.D. Shannon, Acta Cyst. A32,75 1 ( 1976). G.E. Lenain, H.A. McKinstry, J. Alamo and D.K. Agrawal, J. Mater. Sci. 22, 17 (1987). V. Srikanth, E.C. Subbarao, D.K. Agrawal, C.Y. Huang, R. Roy and G. Rao, J. Amer. Ceram. Sot. 74,365 (1991). D.K. Agrawal and V.S. Stubican, Mat. Res. Bull. 20,99 (1985). J.J. Cleveland and R.C. Bradt, J. Amer. Cerum. Sot. 61,478 (1978).